1. Field of the Invention
This invention relates to a zoom lens suitable for a still camera, a video camera, a digital camera, etc., and an image pickup apparatus having the same.
2. Description of Related Art
Recently, with the higher function of an image pickup apparatus (camera) such as a video camera or a digital still camera using a solid state image pickup element, a lens system of a relatively great aperture ratio including a wide angle of field has been required for an optical system used therein.
In a camera of this kind, various optical members such as a low-pass filter and a color correcting filter are disposed between the rearmost portion of a lens and an image pickup element and therefore, a lens system having a relatively long back focal length is required for an optical system used therein.
Further, in the case of a color camera using an image pickup element for a color image, in order to avoid color shading, an optical system having a good image side telecentric characteristic is desired as an optical system used therein.
There have heretofore been proposed various two-unit zoom lenses of a so-called short zoom type having a wide angle of field which comprises two lens units, i.e., a first lens unit of negative optical power and a second lens unit of positive optical power, and effects a focal length change by changing the lens interval between the two lens units. In these optical systems of the short zoom type, the second lens unit of positive optical power is moved to thereby effect a focal length change, and the first lens unit of negative optical power is moved to thereby effect the correction of a fluctuation in the position of an image point resulting from the focal length change. In a lens construction comprising these two lens units, the zoom magnification is of the order of double.
Further, in order to have a double or higher variable power ratio, and yet arrange an entire lens into a compact shape, there has been proposed a so-called three-unit zoom lens in which a third lens unit of negative or positive optical power is disposed on the image side of a two-unit zoom lens and which effects the correction of various aberrations occurring with a higher magnification (for example, Japanese Patent Publication No. H07-3507 and Japanese Patent Publication No. H06-40170).
As the three-unit zoom lens, there is known a three-unit zoom lens system having a wide angle of field which has a long back focal length and satisfies a telecentric characteristic (for example, Japanese Patent Application Laid-open No. S63-135913 and Japanese Patent Application Laid-open No. H07-261083).
Also, in the three-unit zoom lens, there is known a zoom lens in which a first lens unit of negative optical power is made stationary and a second lens unit of positive optical power and a third lens unit of positive optical power are moved to thereby effect zooming (for example, Japanese Patent Application Laid-open No. H03-288113).
Also, in the three-unit zoom lens, there is known a three-unit zoom lens having a relatively small number of constituent lenses in which in case of zooming, all lens units are moved and a cemented lens is effectively used as a second lens unit to thereby correct chromatic aberration (for example, Japanese Patent Application Laid-open No. 2001-272602, Japanese Patent Application Laid-open No. 2003-128261, Japanese Patent Application Laid-open No. 2002-48975, Japanese Patent Application Laid-open No. 2003-5072, Japanese Patent Application Laid-open No. 2003-149555 and Japanese Patent Application Laid-open No. 2003-149556).
Also, in the three-unit zoom lens, there is known a three-unit zoom lens in which the object side surface and image side surface of a lens of negative optical power in a first lens unit are made into an aspherical shape to thereby aim at a further decrease in the number of constituent lenses (for example, Japanese Patent Application Laid-open No. H05-323190, Japanese Patent Application Laid-open No. H07-174971, Japanese Patent Application Laid-open No. 2002-55278 and Japanese Patent Application Laid-open No. 2002-365545).
A three-unit zoom lens designed for a 35 mm film photograph is too long in back focal length and not good in telecentric characteristic for an image pickup apparatus using a solid state image pickup element and therefore, it is difficult to intactly use in an image pickup apparatus.
On the other hand, in recent years, in order to make the compactness of a camera and the higher magnification of a zoom lens compatible, there has been widely used a zoom lens of a so-called flush barrel type in which during non-photographing, the interval between adjacent ones of the lens units is reduced to an interval differing from that in a photographing state to thereby reduce the amount of protrusion of the lens from a camera main body.
Generally, if the number lenses of each lens unit constituting a zoom lens is great, the length of each lens unit on the optical axis thereof becomes great, and if the amount of movement of each lens unit in zooming and focusing is great, the full length of the lens becomes great and a desired flush barrel length cannot become achieved, and this becomes difficult to use in the zoom lens of the flush barrel type.